After several years of modest increases, American spending on medications is projected to shoot up by 12 percent this year, pushing the nation’s drug bill to between $375 billion and $385 billion, according to a report by the IMS Institute for Healthcare Informatics.

Several factors are driving the spending spike, including the introduction of expensive new hepatitis C drugs and fewer drug patent expirations than in previous years, the report found. Such expirations typically lead to savings as cheaper generics replace brand-name drugs.

The 11.7 percent rise is a dramatic departure from the more modest average increases of 3.6 percent in annual drug spending during the past five years.